One-Pipe 3-Circuit Monoflow System

I came across another one today, this one with 3 circuits. If anyone remembers my last thread about a one-pipe system, it had a single abandoned monoflow tee in the return line at the boilers. This one is the same way. A single abandoned monoflow tee in the return.

The supply is 2.5 inch, and the three loops are 2 inch, and all return to a common 2 inch return. Does this seem right?

I came across another one today, this one with 3 circuits. If anyone remembers my last thread about a one-pipe system, it had a single abandoned monoflow tee in the return line at the boilers. This one is the same way. A single abandoned monoflow tee in the return.

The supply is 2.5 inch, and the three loops are 2 inch, and all return to a common 2 inch return. Does this seem right?

Normally, the pipes are sized based on connected load, anticipated delta t at a given flow. If the boiler only has 2" return tapping, then I'm not surprised all combined returns are 2" as well. You'd need to do a reverse load survey, inventorying all radiators connected to a given main before you can determine the proper pipe size.

As for the monoflo tee on the return, hard to say, but my gut tells me its to blend the hotter supply with the cooler return water to keep the boiler from condensing when doing a cold start up.

If the boilers been replaced, this was probably lost in the shuffle...

ME

It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.

I guess to truly figure it out the system would have to be reverse engineered. Each loop would have to have heat loss calculations done.

There are two boilers that aren’t original. They are piped primary secondary with common manifolds. Both rated for 305,000 btu. Cast iron sectionals. With a 20 degree delta T we’d be looking for 61 gpm. Basing that off the boiler output though and they’re likely oversized. Identical to the last monoflow system I posted about but this one has three loops instead of two.

Each boiler has 2 inch tappings. Each loop has a balance valve at the end but they’re all wide open and each loop does see about a 20* delta T. I used Dan’s ballpark friction loss formula and the friction loss through the longest loop pretty much falls right on the pump curve where it should be.

So I’m guessing each loop will be looking for about 20gpm, 200,000 btu/hr. With 2 inch pipe that’s around 1.9 feet per second velocity.

How important is velocity for diverter tees? Does it matter? I guess they’ll still create a pressure drop either way.

I guess they could have dropped the loop pipe sizes down to 1-1/4 but then friction loss would have been about two times greater which would require a larger pump.

So now I’m wondering how the velocity is correlating with the delta T in each loop. If the pipe was reduced to 1-1/4, and a larger pump was installed to overcome the added friction loss, while still providing 20 gpm to each loop, the velocity is going to go up from 1.9 to 4.3. The water is moving faster so it isn’t staying in each convector as long, but 20gpm is 20gpm so the delta T across the loop should stay the same.

Please forgive my random questions... it’s 5am, been running a service call since 3am, now I’m waiting on my next job and as usual my mind is going a million miles per hour. Can’t stop thinking about these questions. I need to seek help. Lol

The system is working fine, I’m just asking these questions as a way of trying to learn.

If it makes you feel any better, I have YET to see a system running at 100% of capacity, even at design conditions, and even on jobs that I did the loss calcs, gain calcs and set the physical plant. Best that I have seen at design condition (bang bang boiler, and pump) is a 50% duty cycle. What this tells me is that there is a WHOLE lot of reality that kicks in and overrides theory, including internal gains, external (solar) gains and what I call the flywheel mass effect.

And FWIW, primary secondary doesn't protect a boiler as well as a non electric TRV (Caleffi, Danfoss/ESBE,Honeywell) will do.

And yes, by doing a heat loss calculation, backed up with a good blower door test, you would be able to fine tune the required water supply temperatures, and may very well result in a significant reduction in discharge temperatures and required flows. With this being standing cast iron radiators, the need for maintaining "sweep velocities" (4 to 5 FPS) in your system fluid are less important. Dirt and air will separate out in the radiation and boilers and not be a problem as it pertains to air "hang up".

ME

It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.

At low velocity, say 2 fps or below the water does not carry the air along as well and you may have trouble purging and keeping good heat transfer. You want water touching steel as much as possible to transfer energy to the load.

Chapter 11 in Modern Hydronic Heating has a good section on divertor tee systems. It takes you step by step from load calc, pipe sizing, and circulator sizing.

The concept of a single pipe system is attractive, but the sizing and balance takes some time.

Bob "hot rod" Rohrtrainer for Caleffi NAThe magic is in hydronics, and hydronics is in me

I read about that 2fps last night. So the pump in question was doing about 60gpm. It splits at the end into three single pipe loops. I have no way of telling the flow through each loop but I’m assuming they would be somewhat equal at around 20gpm each. 2 inch pipe, 20gpm, is less than 2fps. About 1.9. And they do have air problems from what I have heard.

I have Modern Hydronic Heating sitting at home waiting to be dissected! I’ve heard nothing but great things about it. Excited to start reading it.

It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.

Monoflo systems are a royal PITA to purge. Tough to force purge. After you're done force purging, you have to go around to each individual convector, and lift up on the end with the bleeder to get the final little bit of air out. Very time consuming. Really no way around it.

If it were standing radiators, those systems CAN'T be force purged, and require bottom filling with manual venting on top. Also labor intensive, but much ore forgiving if a little bit of air remains behind.

ME

It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.